1. Field of the Invention
This invention generally relates to a ball screw assembly, and, in particular, to improvements in a ball screw assembly.
2. Description of the Prior Art
A ball screw assembly is well known in the art. A typical prior art ball screw assembly includes a screw shaft formed with a spiral groove along its peripheral surface, an outer sleeve fitted onto the screw shaft with a predetermined gap therebetween and a plurality of balls interposed between the screw shaft and the outer sleeve to provide a rolling contact therebetween. The outer sleeve is typically provided with at least one endless circulating path along which the balls circulate endlessly so that the screw shaft may move relative to the outer sleeve in the longitudinal direction. Such a typical ball screw assembly is useful in many applications; however, it tends to be bulky since an endless circulating path must be formed in the outer sleeve, and, moreover, it tends to be expensive due to difficulty in manufacture. Furthermore, since the balls are physically in rolling contact with both of the screw shaft and the outer sleeve, the resistance between the screw shaft and the outer sleeve is relatively large, which tends to hinder a fine relative positional control between the screw shaft and the outer sleeve.
In a typical prior art ball screw assembly, the distance between the outer surface of the screw shaft and the outer surface of the outer sleeve is required to be set approximately at twice the diameter of the balls used, and, thus, if such a ball screw assembly were to be fabricated compact in size, difficulty would be encountered in manufacture since enough space could not be found easily in designing an endless circulating path. Because of recent expansion of applications of robot machines in various fields, there has been a need to develop a ball screw assembly compact in size and high in performance.